Degradation of polymeric materials can be caused by exposure to light, heat, and/or air. Such degradation is usually manifested by either a partial or total loss of structural integrity, changes in light transmission properties, changes in color, loss or reduction in flexibility and/or resiliency, or any combination of the above phenomena. Those attempting to avoid polymer degradation have generally selected from among three possible approaches: elimination or reduction of the degradative forces, isolation of the sensitive polymer material from the degradative forces, or modification of the polymer composition to enhance its resistance to the degradative forces. The latter approach is preferable since it does not require elaborate engineering or structural changes in the polymer environment.
The art of stabilizing polymeric materials is both complex and unpredictable. Ordinarily, the stabilizing agent is physically combined with the polymer thereby prolonging its useful life in the hostile degradative environment. In such cases, the stabilizing agent is generally free to slowly migrate within the composition and to the surface thereof. This presents a problem where the polymer is contacted by fluids which can extract the stabilizing agent. Furthermore, some stabilizing agents are heat sensitive and can change from a solid to a vapor at relatively low temperatures. Exposure of compositions containing such heat sensitive stabilizing agents will result in vaporization of the stabilizer and its eventual diffusion from the polymer. Whether the stabilizer is lost by fluid extraction or vaporization, or by any other means for that matter, the end result is that the polymer will again become vulnerable to attack by degradative forces.